Initial application of novel compound-specific 34S analysis to the Cariaco Basin

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Sulfur is an important component of many biogeochemical processes, spanning atmospheric chemistry and aerosol formation, microbial ecology, ocean anoxia and mass extinctions, organic matter preservation, petroleum generation and souring, and the oxygenation of the ancient Earth's oceans and atmosphere. The stable isotopic compositions of sulfur species (i.e., their dS values) can provide considerable insight in these pursuits, because many redox transformations of sulfur induce large and diagnostic isotopic fractionations. These fractionations can be preserved in sulfur-bearing products, where they serve as fingerprints of past processes. In this regard, organic sulfur (OS) is among the richest "but least utilized" reservoirs of S-isotopic information. Until recently there has been no effective way to access the isotopic information recorded by individual OS compounds. The PIs have recently developed a novel GC/ICPMS analytical approach (Amrani et al., 2009) that now allows us to measure d34S in individual OS compounds containing just picomoles of S. They propose to study the progressive sulfurization of organic compounds during sedimentary diagenesis. Three specific questions will be addressed regarding the sulfurization of lipids: i) Do d34S values of individual compounds record assimilatory versus dissimilatory pathways of sulfur incorporation? ii) Are molecular d34S values offset from their inorganic S sources as a result of fractionations? iii) Do different compounds record sulfide and/or sulfate d34S values from different depths? This proposal will help support the development and deployment of an important new analytical tool, namely compound-specific d34S analysis. The results of their research and the broader potential of the method are likely to impact our understanding of the global carbon cycle, and all of its far-reaching impacts on climate change, because the still poorly understood pathways of sulfurization exert a first-order control on the burial and preservation of organic matter in sediments. The proposal will also support the education of a PhD student at Caltech, who (upon completion) will presumably be the world's expert in compound-specific S isotope analyses and will carry this technique to other labs.